Thermoplastic polyvinyl chloride compounds are used extensively in the industry for producing flexible tubing, pipes, sheeting, tapes and the like and in particular as insulation on electric wires. In this latter capacity the thermoplastic compounds are frequently required to meet certain specifications of industrial groups such as automotive, communications, wire manufacturers and the like, and also certain regulatory groups such as Underwriters' Laboratories, with regard to chemical and/or physical degradation and in particular retention of tensile strength and elongation after aging at elevated temperatures plus resistance to penetration. For example, polyvinyl chloride thermoplastic compounds presently used as insulation for electric wiring have been successful in meeting Underwriters' Laboratories specifications at temperatures as high as 105.degree. C. known commercially as 105.degree. C. wire, this is currently the highest class of PVC thermoplastic insulation approved by U. L. However, there has been a steadily increasing demand in industry for polyvinyl chloride thermoplastic compounds that will resist chemical and/or physical degradation at temperatures above 105.degree. C. and more especially over a much wider temperature range than has been possible using known thermoplastic compositions.
While some work has been done in this area with chlorosulfonated polyethylene thermoset materials and formulations utilizing cross-linked polyvinyl chloride, these compositions present extrusion problems when cross linking is effected during extrusion; or require post-extrusion irradiation treatment.
Of more immediate interest is U.S. Pat. No. 3,824,202 issued July 16, 1974 which discloses the preparation of a polyvinyl chloride thermoplastic composition wherein an alkaline earth metal sulfate filler is used in amount from at least 40 to as high as 90 parts per hundred parts resin the essence of the invention being the discovery that despite the high loadings of filler the PVC thermoplastic material combined good retention of elongation and tensile strength at high temperatures with good penetration resistance. However, these relatively high loadings of filler require high shear conditions during extrusion, i.e., screw compression ratios as high as 3:1 in order to effectively fuse and form extrudates with satisfactory physicals. Also of the alkaline earth metal sulfate fillers mentioned,, barium sulfate, in particular is expensive and its relatively high density gives rise to processing difficulties accompanied by loss of gloss and contour.